reinforcing mesh and two sheets of ply sandwiched together and held to the existing timber work by strips of metal and lots of long coach screws is a nasty combo for anyone making a stealth entry from above

locks are only as good as the door and frame , don't skimp on extra metal for the frame and door, a few sections/plates and fixings can put hours on an entry

go big on fixings, chemical anchors are ace for fixing metal to concrete and always use lots of good quality fastenings

use a combination of "weak" materials to create a strong and difficult to deal with obstacle , if you want to get really nasty a middle layer of chicken wire and glass fibre in the ply and mesh sandwiches is rather fun and very good value for money

bells are good ( as are alarm mines )

any box can be opened but one can make it difficult, unpleasant, noisy and slow

there is "going equipped" but few would be equipped for a proper structure or even if they were they would be unlikely have the time to exploit their knowledge and considerable toolkit